Typical optical data storage media, represented by CDs, DVDs and the like, have transparent protective layers having different thicknesses required for protecting different recording surfaces. First laser beams of a 785 nm wavelength emitted by a first laser beam source are used for reproduction of data on CD-Rs; second laser beams of a 655 nm wavelength emitted by a second laser beam source are used for reproduction of data on DVD. To record and reproduce data on these two types of optical data storage media, a type of optical head apparatus utilizing a common objective lens has been proposed for its capability in converging laser beams on two types of recording surfaces of two types of optical data storage media (e.g., CD and DVD), thereby assisting reduction of size of the apparatus.
Nonetheless, CDs have a protective layer 1.2 mm thick for protecting the recording surface and DVDs have a protective layer of 0.6 mm, which is thinner than 1.2 mm of CDs, but has a higher recording density than CDs. To accommodate this difference, a diffraction grating having microscopic concentric notches is provided to the lens surface of the common objective lens that generates a single refracting power such that incoming laser beams are diffracted by the diffraction grating to form multiple focuses at different points on the optical axis thereof. (See Japanese Unexamined Patent Publication H09-120027, incorporated herein by reference). In this configuration, different orders of diffractions are allocated for the laser beams of the same wavelength, which is an inefficient way of utilizing light. To overcome this problem, another patent proposed a way of increasing efficiency thereof by allocating the same order of diffraction for laser beams of two different wavelengths. In this configuration, the laser beams excellently focus on recording surfaces of DVD and CD (See Japanese Unexamined Patent Publication 2000-81566, incorporated herein by reference).
The above mentioned objective lens of conventional technology is configured in the manner as illustrated in FIG. 11: between incoming end refracting surface 131 and outgoing end refracting surface 132, incoming end refracting surface 131 is divided, for example, into center end refracting surface region 133 having center end diffraction grating 135 and outer circumferential refracting surface region 134 having outer circumferential diffraction grating 136. The above objective lens is configured in the manner illustrated in FIG. 12. To record or reproduce data on CD 41 utilizing first laser beams L1 of 785 nm, diffracted beams obtained via center end refracting surface region 133 form beam spots B41 on the recording surface 41a of CD 41. To record or reproduce data on DVD 42 utilizing second laser beams L2 of 655 nm, first diffracted beams obtained via center end refractive surface region 133 and second diffracted beams obtained via outer circumferential refracting surface region 134 form beam spots B42 on recording surface 42a of DVD 42. Dividing refracting regions as described above allows the divided regions to have different aspheric coefficients or optical path differential functions, thereby optimizing properties for each CD 41 and DVD 42. The overall aberrations are thus minimized.
Generally, center end diffraction grating 135 and outer circumferential diffraction grating 136 can be constructed with multiple notches 135a and 136a shaped in sawteeth (in cross section) having a raised center and depressed outer circumference thereof. This configuration allows the diffracted beams emitted from center end refracting surface region 133 and diffracted beams emitted from outer circumferential diffraction grating 136 to produce (−1) first order diffracted beams. Hence, notches 135a and 136a point to the same direction because it is easier to mold objective lens 130 with notches pointing toward the same direction than objective lens 13 with notches pointing in different directions in view of manufacturing.
Nevertheless, the above objective lens 130 of conventional technology has drawbacks in that a change in temperature causes a change in refractive index and linear expansion in materials that form objective lens 130. In addition, a change in temperature causes a change in wavelength for laser beams, and then, a similar change in both first diffracted beams generated by center end diffraction grating 135 and second diffracted beams generated by outer circumferential diffraction grating 136. Usually, third order spherical aberrations are dependent on temperatures during recording and reproduction of data on DVD 42 which utilizes first diffracted beams generated by center end diffraction grating 135 and second diffracted beams generated by outer circumferential diffraction grating 136. FIG. 13 illustrates the results of a simulation that was carried out utilizing objective lens 130 considering a change in refractive index and linear expansion of materials forming objective lens 130. As is apparent from FIG. 13, the third order spherical aberration changes about 10 mλ at surrounding temperature of −5° C., which is the lowest end; it changes about 30 mλ at 55° C., which is the highest end. In other words, as the surrounding temperature changes ±30° C., the third order spherical aberration changes ±20 mλ.
The height of changes constituting a diffraction grating is 2π, which is the phase of laser beams. However, it is first laser beams of 785 nm that is used for reproduction and recording of data on CDs and second laser beams of 655 nm that is used for reproduction and recording of data on DVDs that enter the diffraction grating. To accommodate these beams, heights of notches of diffraction gratings are set to h1 and h2, and are obtained by the following equations:h1=λ1/(n1−1)h2=λ2/(n2−1)
Alternatively, the height of notches of a diffraction grating may be set to (h1+h2)/2;
wherein n1 is the refractive index of the objective lens for first laser beams L1; n2 is the refractive index of the objective lens for second laser beams L2; λ1 is a wavelength of first laser beams; and λ2 is a wavelength of second laser beams.
FIG. 14(a) illustrates the S-curve properties (focusing error signal) of data taken under the condition that second laser beams are given priority over first laser beams and the height of notch for the diffraction grating is set to h2. The resulting resolution expressed by the S-curve is excellent for DVDs but poor for CDs. Additionally, the resulting jitter levels representing data reproduction performance are excellent for DVDs, but only at the lowest level required by the CD specification.
In contrast, FIG. 14(b) illustrates the S-curve properties (focusing error signal) of data taken under the condition that first laser beams are given priority over second laser beams and the height of notch for a diffraction grating is set to h2. The resulting resolution expressed by the S-curve is excellent for CDs but the amplitude thereof is too small for DVDs to accurately pick up focusing servo during data recording and reproduction of a dual layer disk. Additionally, a disk contaminated with fingerprints (e.g., fingerprint disk) may provide erroneous focusing servo. Moreover, the resulting jitter levels representing data reproduction performance are excellent for CDs, but are out of specification for DVDs, which does not allow DVDs to reproduce data thereon.
FIG. 14(c) illustrates the S-curve properties when the height of notch of a diffraction grating is (h1+h2)/2. The resulting resolution during data reproduction is as good as that shown in FIG. 14(b) for a CD but it is poor for DVDs than CDs even though DVDs require better resolution than CDs during data reproduction.
Alternatively, the refracting surface of the objective lens may be divided into the center end and the outer circumference, and the center end diffraction grating having notch of (h1+h2)/2 high may be provided at the outer circumference thereof such that beams diffracted by the center end diffraction grating are used for reproduction of data on a CD utilizing first laser beams while beams diffracted by the outer circumferential diffraction grating are used for reproduction of data on a DVD utilizing second laser beams. Conventionally, the height of notch for the outer circumferential diffraction grating is h2 to increase efficiency of second laser beams. However, the inventors found that the use of the above configuration causes displacement in phases for those second laser beams diffracted by the center end diffraction grating and those diffracted by the outer circumferential diffraction grating. Frontwave aberrations thus deteriorate, thereby providing poor transmittivity for the overall lens.
Further, in the above objective lens of conventional technology, aberrations are eliminated by optimizing the shape of aspheric surfaces of the incoming end refracting surface and the outgoing end refracting surface in accordance with the wavelength of each laser beam. As a result, once centering on the incoming refracting surface and the outgoing end refracting surface fails, focusing on the recording surface of an optical data storage media fails. For example, FIG. 15(a) illustrates the relationship between parallel eccentricity at incoming end refracting surface and at outgoing end refracting surface and the third order spherical aberration (solid line L11), coma aberration (dashed line L12), aspheric aberration (one-dot broken line L13), and wavefront aberration (two-dot broken line L14). An increase in parallel eccentricity causes a significant deterioration in coma aberration and wavefront aberration, necessitating unrealistic accuracy for a metallic cast which allows mass production of the lens of this type.
When the objective lens tilts due to tilt controlling, laser beams do not focus well on the recording surface of an optical data storage medium. For example, FIG. 10(b) illustrates the relationship between tilting and the third order spherical aberration (solid line L1), coma aberration (dashed line L2), aspheric aberration (one-dot broken line L3), and wavefront aberration (two-dot broken line L4). As the objective lens tilts, coma aberrations and front wave aberrations significantly deteriorate, which is a problem.
In view of the previously described problems, the objective of the invention is to provide a diffractive lens configuration with an improved temperature property (resistance) for a lens capable of diffracting first and second laser beams having different wavelengths. It also provides an optical head apparatus that utilizes the lens as an objective lens.
Another objective of the present invention is to provide an optical head apparatus and an objective lens for the optical head apparatus that has excellent pick up properties even though first and second laser beams having different wavelengths are condensed on the recording surfaces of transparent protective layers of different thicknesses covering first and second optical data storage media via the objective lens having a diffractive lens structure.
Yet another objective of the present invention is to provide an optical head apparatus and an objective lens for the optical head apparatus that has excellent pick up properties even though the optical axes at the center of an incoming end refracting surface and at the outgoing end refracting surface of a lens having a diffractive lens structure are somewhat displaced in the direction perpendicular to the optical axes, or the optical axes are tilted.